The present invention relates to the growth of single-crystal Aluminum Nitride (AlN), and more particularly, to relatively large, single-crystal AlN, which are grown by sublimation-recondensation at growth rates exceeding 0.5 mm/hr.
Status of III-Nitride Technology Using Commercially Available Substrates
Several types of materials are routinely used to form semiconductor substrates. Sapphire is popular, because relatively high-quality, inexpensive sapphire substrates are commercially available. However, sapphire is far from being an ideal substrate for GaN epitaxy. Its lattice mismatch to GaN is large (about 16%), it has little distinction between the + and xe2x88x92[0001] directions which can give rise to +/xe2x88x92c-axis domains in epitaxial films of GaN, and its differential thermal expansion can lead to cracking during the cooling process after the device fabrication process. In spite of those problems, recently, Nichia Ltd (Japan) has announced the production of the first violet laser with commercial possibilities (more than 10,000 hours of operating life) using sapphire substrates. Currently, LDs (Laser Diodes) are selling for around $2,000 apiece. Using sapphire substrates leads to a costly fabrication process since it requires growing buffer layers and using Lateral Epitaxial Overgrowth techniques (LEO). Even though this announcement is very promising, Nichia""s lasers still have problems. Some sources claim that heat builds up in these lasers as they shine. Sapphire, with a very low thermal conductivity, traps that heat, a fault that may trigger burnout down the road. To build an even more durable blue laser, Nichia and others are investigating other alternatives such as free-standing substrates. In this technique, the substrate is removed after a thick GaN layer is grown atop the sapphire. This method leaves the GaN as the base for building the laser. This base should be better at dissipating heat, in addition to matching the alloy layers above. However, this alternative may increase fabrication cost.
Single-crystal substrates of SiC are attractive due to their close lattice match to AlN/GaN in the plane perpendicular to the c-axis (the so-called c-plane) and high thermal conductivity. In addition, SiC substrates can be made electrically conducting, which is attractive for some applications (such as LEDs and LDs). However, 2H SiC (to match the 2H crystal structure of GaN) is not available and the lattice mismatch along the c-axis between GaN and both 4H and 6H SiC is substantial. In addition, the chemical bonding between the Group IV elements of the SiC and the Group-III or Group-V elements of the nitrides is expected to create potential nucleation problems leading to electronic states at the interface.
For devices that use GaN or Ga1xe2x88x92xInxN, nominally the most desirable substrate would be large area GaN single crystal wafers. Several methods to grow bulk GaN crystals have been proposed. While this possibility has become more attractive in the last few years, it does not appear to be commercially feasible to fabricate large bulk crystals of GaN in the short term.
It may therefore be desirable to provide alternative substrates such as AlN, for fabricating nitride-based (e.g., GaN) commercial devices. A sublimation-recondensation technique was developed for AlN crystal growth by Slack and McNelly (G. A. Slack and T. McNelly, J. Cryst. Growth 34, 263 (1976) and 42, 560 (1977), hereinafter the xe2x80x9cSlack referencexe2x80x9d). In this technique, polycrystalline source material is placed in the hot end of a crucible while the other end is kept cooler. The crystal nucleates in the tip and grows as the crucible is moved through the temperature gradient. This approach demonstrated relatively slow crystal growth of 0.3 mm/hr while the crystal growth chamber was maintained at 1 atm (100 kPa) of N2. To make such substrates commercially feasible, it would be desirable to increase the growth rate. A number of researchers skilled in art have examined the possibility.
However, most artisans in this field have based their work on rate equations derived by Dryburgh (Estimation of maximum growth rate for aluminum nitride crystals by direct sublimation, J. Crystal Growth 125, 65 (1992)), which appear to overestimate the growth rate of AlN and, in particular, suggest that the maximal growth conditions are near stoichiometric vapor conditions, i.e., the Al and N2 partial pressures should be adjusted so that the Al partial pressure is twice that of the N2. This for example, is a significant teaching of U.S. Pat. Nos.: 5,858,085; 5,972,109; 6,045,612; 6,048,813; 6,063,185; 6,086,672; and 6,296,956; all to Hunter. In addition, the art teaches that the N2 partial pressure should be maintained at less than atmospheric pressure.
Unfortunately, however, most attempts at increasing the growth rate of AlN crystals under such stoichiometric and/or sub atmospheric pressure conditions have met with limited success. In addition, it appears to be impossible to achieve the growth rate, or the electronics-grade quality Hunter discloses in his patents with the nitrogen pressure below one atmosphere.
Additional AlN work was completed by Segal et al. (A. S. Segal, S. Yu. Karpov, Yu. N. Makarov, E. N. Mokhov, A. D. Roenkov, M. G. Ramm, Yu. A. Vodakov, xe2x80x9cOn mechanisms of sublimation growth of AlN bulk crystals,xe2x80x9d J. Crystal Growth 211, 68 (2000)). While this work was published subsequently to the conception of the present invention, it appears to be the first peer reviewed publication to suggest that Dryburgh""s growth equations are incorrect. Segal et al., however, teach growth conditions and experiments that are open, which allows the Al vapor to escape. Disadvantageously, it would be difficult to grow large boules of AlN this way since: (i) control of growth would be difficult (since it would be non-uniform across the surface), (ii) a large amount of Al would be wasted, (iii) the excess Al in the rest of the furnace would create problems because of its high reactivity, and (iv) it would be difficult to maintain high differences in temperature (T) between the source and growing crystal surface.
A need therefore exists for electronics-grade AlN substrates that address the aforementioned drawbacks.
An aspect of the invention includes an apparatus for the growth of bulk single crystal aluminum nitride. The apparatus includes a housing defining a growth chamber, the housing including a gas outlet configured for selectively evacuating and venting the growth chamber, a gas inlet configured for pressurizing the growth chamber, and a viewing port configured for pyrometric monitoring of crystal growth temperatures within the growth chamber. A radio frequency (rf) coil is disposed within the growth chamber and configured for inducing an electromagnetic field therein. A quartz tube is disposed coaxially within the coil. A first set of shielding is disposed coaxially within the quartz tube, including from about 5 to about 7 concentric pyrolytic boron nitride (pBN) cylinders, each of the pBN cylinders having a wall thickness of greater than about 0.05 inches (0.13 cm), each of the cylinders having a length dimension along the longitudinal axis greater than the length dimension of the coil. A second set of shielding is disposed coaxially within the first set of shielding, the second set of shielding including two concentric, open joint tungsten cylinders, each of the tungsten cylinders having a wall thickness of less than about 0.005 inches (0.013 cm); each of the tungsten cylinders having a length dimension along the longitudinal axis less than the length dimension of the rf coil. A push tube is disposed coaxially within the second set of shielding; the push tube having a proximal side and a distal side, the distal side including a set of metallic baffles having a center hole which provides for the pyrometric monitoring of crystal growth temperatures, the proximal side including another set of metallic baffles. A crucible is disposed coaxially within the push tube, the crucible having a conically shaped distal end and a proximal end; the crucible defining a crystal growth enclosure; the proximal end including a high purity, polycrystalline aluminum nitride source material, the distal end being configured for growth of the bulk single crystal aluminum nitride. The push tube is disposed on a push rod assembly configured for sliding the crucible and the push tube along the longitudinal axis. The first set of shielding and the second set of shielding are configured to provide a thermal gradient axially within the cavity of the crucible of greater than about 100xc2x0 C./cm.
Another aspect of the invention includes a method for growing bulk single crystals of aluminum nitride. The method includes utilizing the apparatus described above, purging the growth chamber by evacuating the growth chamber to a pressure less than or equal to about 0.01 mbar (1 Pa), and refilling the growth chamber with substantially pure nitrogen gas to a pressure of about 1 bar (100 kPa). The growth chamber is then evacuated to a pressure less than or equal to about 0.01 mbar (1 Pa), and then pressurized to about 1 bar (100 kPa) with a gas including about 95% nitrogen and about 5% hydrogen. The chamber is heated to a first temperature, the heating including ramping the temperature of the conical upper end of the crucible to about 1800xc2x0 C. in a period of about 15 minutes. The growth chamber is then pressurized to about 1.3 bar (130 kPa) with the gas including about 95% nitrogen and about 5% hydrogen, and heated to a growth temperature. A distal end of the crucible is then ramped to about 2200xc2x0 C. in a period of about 5 hours. The push tube and the crucible are moved axially through the growth chamber at a rate of about 0.6 to about 0.9 millimeters per hour, wherein single crystals of aluminum nitride are grown.
A further aspect of the invention includes a method for growing bulk single crystals of aluminum nitride. The method includes evacuating a growth chamber, pressurizing the growth chamber to about 1 bar with a gas including about 95% nitrogen and about 5% hydrogen, and placing source polycrystalline AlN in a proximal end of a crystal growth enclosure. The method further includes placing a distal end of the crystal growth enclosure in a high temperature region of the growth chamber, ramping the high temperature region to about 1800xc2x0 C., maintaining pressure in the growth chamber at about 1.3 bar, and ramping the high temperature region to about 2200xc2x0 C. The distal end of the crystal growth enclosure is moved towards a low temperature region of growth chamber at a rate of about 0.6 to about 0.9 millimeters per hour, wherein a single crystal of aluminum nitride grows at the distal end of the crystal growth enclosure.
A still further aspect of the invention includes a method of producing bulk single crystals of AlN, which includes providing in a crystal growth enclosure Al and N2 vapor capable of forming bulk crystals, maintaining in the crystal growth enclosure a N2 partial pressure which is greater than the stoichiometric pressure relative to the Al, maintaining the total vapor pressure in the crystal growth enclosure at super-atmospheric pressure, and providing at least one nucleation site in the crystal growth enclosure. The method also includes cooling the nucleation site relative to other locations in the crystal growth enclosure, and depositing the vapor under conditions capable of growing single crystalline AlN originating at the nucleation site. A variation of this aspect includes producing a prepared substrate by cutting a wafer or a cylinder from the bulk single crystal; preparing a surface on the wafer or cylinder receptive to an epitaxial layer; and depositing an epitaxial layer or a complete crystal boule on the surface.
The invention also includes a system for producing bulk single crystals of AlN. The system includes a source of Al and N2 vapor, a crystal growth enclosure for containing the vapor, and at least one nucleation site in the crystal growth enclosure. The crystal growth enclosure has a selective barrier configured to permit migration of N2 therethrough, and to substantially prevent migration of Al therethrough. A pressurization system is configured to maintain in the crystal growth enclosure, a N2 partial pressure greater than stoichiometric pressure relative to the Al, and to maintain the total vapor pressure in the crystal growth enclosure at super-atmospheric pressure. A selective heating system is configured to maintain the nucleation site at a temperature lower than at other locations in the crystal growth enclosure.